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Astronomical spectroscopy

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High resolution spectrum of the Sun showing thousands of elemental absorption lines ().
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High resolution spectrum of the Sun showing thousands of elemental absorption lines (fraunhofer lines).

Astronomical spectroscopy is the technique of spectroscopy used in astronomy. As spectroscopy is described in its own article, this article focuses on its use in astronomy. The object of study is the spectrum of electromagnetic radiation, including visible light, which radiates from stars and other celestial objects. Spectroscopy can be used to derive many properties of distant stars and galaxies, such as their chemical composition and also their motion, via the Doppler shift.

Contents

Stars

Astronomical spectroscopy began with Isaac Newton's initial observations of the light of the sun, dispersed by a prism. He saw a rainbow of colour, and may even have seen absorption lines. These dark bands which appear throughout the solar spectrum were first described in detail by Joseph von Fraunhofer. Most stellar spectra share these two dominant features of the sun's spectrum: emission at all wavelengths across the optical spectrum (the continuum) with many discrete absorption lines superimposed on top.

Fraunhofer and Angelo Secchi were among the pioneers of spectroscopy of the sun and other stars. Secchi is particularly noted for classifying stars into spectral types, based on the number and strength of the absorption lines in their spectra. Later the origin of the spectral types was found to be related to the temperature of the surface of the star: particular absorption lines can be observed only for a certain range of temperatures; because only in that range are the involved atomic energy levels populated.

The absorption lines in stellar spectra can be used to determine the chemical composition of the star. Each element is responsible for a different set of absorption lines in the spectrum, at wavelengths which can be measured extremely accurately by laboratory experiments. Then, an absorption line at the given wavelength in a stellar spectrum shows that the element must be present. Of particular importance are the absorption lines of hydrogen (which is found in the atmosphere of nearly every star); the hydrogen lines within the visual spectrum are known as Balmer lines.

In 1868, Sir Norman Lockyer observed strong yellow lines in the solar spectrum which had never been seen in laboratory experiments. He deduced that they must be due to an unknown element, which he called helium, from the Greek helios (sun). Helium wasn't conclusively detected on earth until 25 years later.

Also in the 1860s, emission lines (particularly a green line) were observed in the coronal spectrum during solar eclipses that did not correspond to any known spectral lines. Again it was proposed that these were due to an unknown element, provisionally named coronium. It was not until the 1930s that it was discovered that these lines were due to highly-ionised iron and nickel, the high ionisation being due to the extreme temperature of the solar corona.

In conjunction with atomic physics and models of stellar evolution, stellar spectroscopy is today used to determine a multitude of properties of stars: their distance, age, luminosity and rate of mass loss can all be estimated from spectral studies, and Doppler shift studies can uncover the presence of hidden companions such as black holes and exoplanets.

Nebulae

In the early days of telescopic astronomy, the word nebula was used to describe any fuzzy patch of light that didn't look like a star. Many of these, such as the Andromeda Nebula, had spectra that looked in many ways a lot like stellar spectra, and these turned out to be galaxies. Others, such as the Cat's Eye Nebula, had very different spectra. When William Huggins looked at the Cat's Eye, he found no continuous spectrum like that seen in the sun, but just a few strong emission lines. These lines did not correspond to any known elements on earth, and so just as helium had been identified in the sun, astronomers suggested that the lines were due to a new element, nebulium (occasionally nebulum or nephelium). In fact, it was discovered in the 1920s that the lines were due to oxygen, a very familiar element. But nebulae are typically extremely rarefied, much less dense than the hardest vacuum ever produced on earth. In these conditions, atoms behave quite differently and lines can form which are suppressed at normal densities. These lines are known as forbidden lines, and are the strongest lines in most nebular spectra.

Galaxies

The spectra of galaxies look somewhat similar to stellar spectra, as they consist of the light from millions of stars combined. Galactic spectroscopy has led to many fundamental discoveries. Edwin Hubble discovered in the 1920's that, apart from the nearest ones (the Local Group), all galaxies are receding from the earth. The further away a galaxy, the faster it is receding (Hubble's Law). This was the first indication that the universe began in a Big Bang (but see also Steady state theory).

Doppler shift studies of clusters of galaxies by Fritz Zwicky found that most galaxies were moving much faster than seemed to be possible, from what was known about the mass of the cluster. Zwicky hypothesised that there must be a great deal of non-luminous matter in the galaxy clusters, which became known as dark matter.

Quasars

In the 1950s, some strong radio sources were found to be associated with very dim objects that seemed to be very blue. These were named Quasi-stellar radio sources, or quasars. When the first spectrum of one of these objects was taken, it was something of a mystery, with absorption lines at wavelengths where none were expected. It was soon realised that what was being seen was a normal galactic spectrum, but highly redshifted. According to Hubble's Law, this implied that the quasar must be extremely distant, and therefore highly luminous. Quasars are now thought to be galaxies forming, with their extreme energy output being powered by super-massive black holes.

Planets and asteroids

Planets and asteroids shine only by reflecting the sun's light. The reflected light contains absorption bands due to minerals in the rocks present for rocky bodies, or due to the elements and molecules present in the atmospheres of the Gas giants. Asteroids can be classified into three main types, according to their spectra: the C-types are made of carbonaceous material, S-types consist mainly of silicates, and M-types are 'metallic'. C- and S-type asteroids are the most common.

Comets

The spectra of comets consist of a reflected solar spectrum from the dusty clouds surrounding the comet, as well as emission lines formed when the solar wind collides with gases surrounding the comet. Analysis of the composition of comets has shown that they are made of pristine material from the earliest days of the solar system. Many organic chemicals are known to exist in comets, and it has been suggested that cometary impacts provided the Earth with much of the water for its oceans and the chemicals necessary for the formation of life. It has even been suggested that life may have been brought to earth from interstellar space by comets (the Panspermia theory).

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